The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident actinic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge after a suitable period of dark adaptation. The element is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of the surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or in the absence of charge pattern as desired. The deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor and the like or transferred to a second element to which it may similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.
Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder have found wide application in present-day document copying applications.
Since the introduction of electrophotography, a great many organic compounds have been found to possess some degree of photoconductivity. Many organic compounds such as poly(vinyl carbazole) have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Optically clear organic photoconductor-containing elements having desirable electrophotographic properties can be especially useful in electrophotography. Such electrophotographic elements may be exposed through a transparent base, if desired, thereby providing unusual flexibility in equipment design. Such compositions when coated as a film or layer on a suitable support also yield an element which is reusable; that is, it can be used to form subsequent images after residual toner from prior images has been removed by transfer and/or cleaning.
Although many of the organic photoconductor materials are inherently light sensitive, their degree of sensitivity is usually low so that it is often necessary to add materials to increase their speed. Increasing the electrophotographic speed has several advantages in that it reduces exposure time, allows projection printing through various optical systems, etc. By increasing the speed through the use of sensitizers, photoconductors which would otherwise have been unsatisfactory are useful in processes where higher speeds are required. Accordingly, there is a need for new materials useful as sensitizers of organic photoconductor-containing systems.
Pyrylium salts, as disclosed in Davis et al, U.S. Pat. No. 3,141,770 issued July 21, 1964 and in VanAllan et al, U.S. Pat. No. 3,250,615 issued May 10, 1966, have been found to be useful sensitizing compounds for photoconductive compositions, especially organic photoconductive compositions. Since the initial discoveries of Davis et al and VanAllan et al as set forth in the above-referred to patents, a variety of new individual species of pyrylium salts, as well as thiapyrylium and selenapyrylium salts, have been discovered and found to be useful as radiation sensitive addenda in photoconductive compositions as well as in other radiation senstitive compositions such as laser Q-switches and various types of filter elements such as infrared absorbing filter elements. For example, Reynolds and VanAllan in U.S. Pat. No. 3,417,083 issued Dec. 17, 1968 disclose the use of certain new stable polymethine pyrylium and thiapyrylium salts useful as laser Q-switches and as infrared absorbing dyes useful in various filter applications. In addition to the various polymethine pyrylium and thiapyrylium dye structures shown in U.S. Pat. No. 3,417,083, Reynolds et al in U.S. Pat. No. 3,938,994 issued Feb. 17, 1976 discloses the use of certain monomethine pyrylium and thiapyrylium salts useful as sensitizers in an organic photoconductive composition.
Because of the especially useful properties which have been found to be associated with various pyrylium, thiapyrylium, and selenapyrylium salts, much work has been done pertaining to the synthesis of pyrylium salts to discover further new and useful species of these materials. In this regard, for background purposes, reference may be made to those species of pyrylium salts disclosed, for example, in Contois et al, U.S. Pat. No. 3,586,500 issued June 22, 1971 and in Contois U.S. Pat. No. 3,577,235 issued May 4, 1971. Still other species of pyrylium salts which have been found to be useful as sensitizers for low-color photoconductive compositions are disclosed in VanAllan, U.S. Pat. No. 3,554,745 issued Jan. 12, 1971. Still other species of pyrylium salt materials are disclosed in Defensive Publications Nos. T889,021; T889,022; and T889,023; all issued on Aug. 31, 1971. Yet other useful species of pyrylium salt materials are disclosed in Belgium Pat. No. 754,066 dated Sept. 30, 1970 and in Reynolds et al, U.S. patent application Ser. No. 60,634 filed Aug. 3, 1970 and now abandoned.